We had the same problem, but with (the neighbors) cats instead of squirrels. They would dig up our mulch, use our new lawn as their litter box, etc., stuff like that. Home Depot sells many different kinds of animal repellent sprays that are plant safe, and won't harm the animals, just repel them. And they work!!! However, the instructions say spray once a month or so, don't pay attention to that, you need to be vigilent and spray often. And check the labels; different formulas for different kinds of animals. Good luck! And let me know how it works out for you.

I have used cayenne pepper sprinkled on top of the soil and I have also put small rocks on top of the soil.